1. Field of the Invention
The present invention relates to a pixel sensor that has a linear response to low light intensity and a logarithmic response to high light intensity. In particular, the invention relates to the use of a reset gate to pre-bias an accumulation node of the pixel so as to cut-off current through a logarithmic response transistor and thereby provide a linear response region.
2. Description of Related Art
A standard log response pixel is based on the notion of sub-threshold conduction. In FIG. 7, a gate of the field effect transistor (FET), denoted Q.sub.1, is connected to a first predetermined potential, denoted V.sub.DD. In this configuration, the voltage at node 1, denoted 1, will have a logarithmic dependence on a photon-induced signal current from the photo-diode. In practice, the voltage output to the readout portion tracks the input illumination and thus this pixel does not require a reset FET. The equivalent resistance through the FET denoted Q.sub.1 permits charge to build up and diminish on the parasitic capacitance at node 1 (e.g., due to readout structure). This design achieves a dynamic range of many orders of magnitude. Since the human eye has the same or similar log response, the log response of the pixel is a desired characteristic in electronic vision systems.
In FIG. 8, another known log pixel (referred to as the Chamberlain pixel) connects the gate of transistor Q.sub.1, to the transistor's source (instead of its drain). See Chamberlain and Lee, "A Novel Wide Dynamic Range Silicon, Photodetector and Linear Image Array", IEEE Journal Of Solid-State Circuits, Volume SC-19, No. 1, February 1984, pages 41-48. This pixel works on a sub-threshold logarithmic voltage response to current, and it is described in U.S. Pat. No. 4,473,836 granted to Chamberlain, incorporated herein by reference.
In FIG. 9 and in U.S. Pat. No. 4,794,247 to Stineman, Jr. there is disclosed photo-diode 201 coupled to integrating amplifier 203, 230 with reset transistor 202 to preset the integration constant. However, this pixel does not include a transistor in series with photo-diode 201 to provide the pixel with a log response region.
In FIG. 10 and in U.S. Pat. No. 5,742,047 to Buhler, et al. there is disclosed a photo-diode D1 coupled to a diode reset transistor M1 and, through a pass transistor M2, coupled to a "fat zero" transistor M3 where an integration node is at the junction between M2 and M3. However, this pixel does not include a transistor in series with photo-diode 201 to provide the pixel with a log response region.
The pixel of FIG. 7 or FIG. 8 typically produce signal swings (in response to light to dark swings) in the order of 100 millivolts. With such small signals, it is difficult to distinguish photon induced signals from dark current and other noise signals, and therefore, it is difficult to achieve high signal to noise sensors, especially a low light levels.
Furthermore, the signal response time to a photon signal input can be long, especially at low light levels where the photo-current is small. The effective resistance across the FET is large at low light levels, and the time constant to achieve equilibrium of the voltage stored on the parasitic capacitance at node 1 is long. This leads to an image lag phenomenon called ghosting (e.g., where a brightly illuminated pixel takes a prolonged time to output a dark signal when the bright illumination is removed).